The photovoltaic (PV) industry is undergoing rapid growth with shipments of more than 2.5 GW of PV modules in 2006. This represents about a 40% growth rate for last year, and it is projected that worldwide shipments of PV modules will increase by about 40-50% per year rate to over 10 GW (giga watts) per annum in 2010. Therefore, the PV industry has grown steadily to a critical size to be considered as an important industry. More than 90% of worldwide production of PV modules uses crystalline silicon; amorphous silicon and thin film technologies constitute less than 10% of the production with the majority of it for consumer electronics applications.
Even though the PV industry has matured, it has relied on the electronic industry for its silicon feedstock. Up until recently, excess capacity, rejects and scraps from the electronic industry were used as feedstock. For the electronic industry, the cost of silicon wafers is less than 5% of the device cost, whereas for the PV industry it is about 30% of the module cost. Therefore, it is important to have a low-cost solar grade (SoG) silicon feedstock for the PV industry. If an independent low-cost source of SoG silicon becomes available, the growth of the PV industry can increase at a faster rate.
The rapid growth of the PV industry, absence of an independent source of SoG silicon and reluctance to add additional capacity by major electronic grade silicon feedstock manufacturers has created a shortfall in the supply of silicon feedstock for the PV industry. Therefore, the PV industry has had to resort to using lower and lower grades of silicon feedstock to meet its needs. Fortunately, the purity of silicon feedstock for solar cells is more forgiving as compared to most microelectronic devices.
Silicon feedstock for electronic applications (EG silicon feedstock) utilizes extensive chemical processing. With this approach, all impurities are reduced to <1 ppba level. Most solar cell processing requires silicon with about 0.5 ppma boron (B). Therefore, B dopant is added when EG silicon feedstock is used. It is also recognized that high efficiency solar cells can be produced even when metallic impurities are in the 0.1 ppma range. In view of these criteria, the SoG silicon feedstock can contain higher levels of impurities than EG silicon feedstock without compromising solar cell performance. In addition to metallic impurities, solar cells are also somewhat tolerant to finely dispersed secondary phases, such as, oxides, carbides, etc.
The production of EG silicon feedstock is by reduction of chloro-silanes. The most commonly used process involves reduction of trichlorosilane in a Siemens reactor. Another approach is silane reduction in a fluidized bed reactor (FBR). The latter approach offers lower production costs and is, therefore, attractive for large volume production especially for PV applications. The FBR produces a more desirable bead size product compared to the large chunk size product from the Siemens reactor. Although most of the silicon is produced as beads in the fluidized bed, approximately 10% of the silicon is deposited in a very fine powder form on the walls of the reactor. A smaller amount of silicon powder is also carried by the outgoing gas stream and is trapped in the on-line filters. Chemically, these silicon powders are similar, with respect to the metallic impurities, to the silicon beads product but these powders range from submicron size to a few micron sizes. The large surface area of the powders (due to fine particle size) can lead to surface contamination from deposition on the surface, handling and exposure to the ambient environment. Typically, transition metal impurity contamination and oxidation of the surface can be expected. When such fine powders are used as wafers for an ingot growth process (such as multicrystalline ingot production) many problems are encountered, for example, (1) poor packing density of the powder in the crucible, (2) transport of the powder from the crucible into the heat zone during evacuation, (3) reaction of the powder in the hot zone to form carbides, (4) problems with melting if sufficient higher melting oxides/carbides are formed, (5) separation of the oxide/carbide slag from the silicon melt, (6) reduction of impurities during the solidification of the silicon, and (7) utilization of the product for PV applications. The present invention relates to resolving or reducing some of the problems encountered in using such silicon powder.
For several years efforts have been made to utilize the fine powder that has been a by-product from the FBR for PV applications. When attempts were made to melt these powders in resistively heated furnaces, it was difficult to even melt the charge at temperatures in excess of the melting point of silicon (1412° C.). This was attributed to (i) an oxide layer covering the silicon particles due to exposure to air or (ii) to carbide formation during heating in the furnaces. Both silicon oxide and silicon carbide have significantly higher melting temperatures than silicon. When attempts were made to mix the powders with bulk silicon feedstock, the solar cell performance was degraded. Hence most of the powder ended up being used by the metals industry rather than for electronic devices.
In order to extend the supply of silicon feedstock in the current shortfall situation, utilization of these powders has been re-examined. It is generally agreed that, if the powders comprise more than about 5% of the feedstock charge by weight, serious degradation of solar cell performance can be expected. Therefore, it has been debated whether it is worth risking 95% of the bulk material in this shortage environment for silicon feedstock because some of the powders have been contaminated in their handling as a by product of the FBR bead production.